The present invention relates especially to an improvement in a carbide drill for deep hole machining.
It is well known that in a carbide drill of this type, when two tips are attached to the drill head, the chisel edge (the very central point of the cutting edge) which does not carry out the machining function appears at the central point of rotation of the drill head and that accordingly the central portion of the workpiece is forcibly crushed around the central point of rotation of the drill head in the course of drilling work. Therefore, it has been believed that a two-tip carbide drill is suitable for drilling work on relatively soft nonferrous metals such as cast iron or aluminum, but not suitable for drilling work on hard metal such as steel materials because the tips are easily damaged due to great resistance generated when the workpiece is crushed and removed as well as great thrust resistance. As a result, a single-tip carbide drill in which the chisel edge does not appear has been used in processing of steel materials.
Even in a single-tip carbide drill, however, if a single tip is attached to the drill head in a radial direction with respect to the axis of rotation, the cutting edge must travel exactly on the axis of rotation as a matter of course in order to machine the workpiece up to its central portion. Therefore, in the manufacture of the drill, strict accuracy is required for attaching the tip. Even if the tip may be attached exactly, a load is imposed on the portion of the cutting edge which corresponds to the central point of the drill by the thrust resistance as the cutting speed at the central point of the drill is theoretically zero, which results in the fact that the tip is easily damaged and the machining ability cannot be increased. Moreover, a single-tip carbide drill is inferior to a two-tip carbide drill in its processing efficiency because of its lower amount of machining and feed amount.
Over against this, it has been proposed recently to provide a two-tip drill intentionally with a nonmachining zone between the two tips attached to the drill head in order to resolve said disadvantages of a two-tip drill. FIG. 13 shows an example of such a drill. This conventional structure is such that tips K1 and K2 are fixed to the drill head respectively in a radial direction with the axis of rotation O of the drill head H between and an opening of some 0.5 mm. in width, namely, a nonmachining zone Z is provided between both tips K1 and K2. It is believed that in performing machining through drilling, a core C is generated since, as shown in FIG. 12, the part of the workpiece which corresponds to the nonmachining zone Z is not machined as a matter of course but this core will not constitute a hindrance to the drilling work, because this core C is of an insignificantly small size which is generated in an opening of some 0.5 mm., so that said core will repeat its growth and disconnection from the workpiece itself in the course of machining to be taken away together with chips.
In this proposed conventional structure, machining does not include forcible crushing of the workpiece because the cutting edge (chisel edge) does not appear, and the thrust resistance and the damage of the tip can all the more be reduced. In some sorts of workpieces, however, the growing core is too strong to be removed because it is constructed such that the core C of the workpiece which grows in the nonmachining zone Z drops out by itself to be removed in the course of machining. In the case of a core of a larger diameter, it cannot be removed, either. Accordingly, it is not possible to make the width of the nonmachining zone Z large. Consequently, in the drill of this conventional type, dropping out of the core C is uncertain and there is no guarantee of the core C's dropping out surely in the course of machining.